The long-term goal of this project is to elucidate the cellular mechanisms involved in modulation of the estradiol-induced preovulatory LH surge by progesterone or nonsteroidal antiestrogens. The observation that progesterone or synthetic progestins could block gonadotropin surges and ovulation provided the basis for the development of modern antifertility drugs, and the antiestrogens are also of clinical importance for treatment of infertility. Conditions have been established in this laboratory for demonstration of inhibition of estradiol-induced LH surges by progesterone or antiestrogens (nafoxidine or 4-hydroxytamoxifen) or for facilitation of these surges by progesterone in the immature (28-30 day-old) female rat. These paradigms will be employed to examine the possible effects of progesterone or hydroxytamoxifen on the following parameters: (1) steady-state levels of messenger RNA for the rat alpha and LH beta subunits using specific cDNA probes corresponding to portions of these genes and RNA blot hybridization; (2) changes in transcription of the genes encoding the rat alpha and LH beta subunits using nuclear runoff (in vitro) transcription assays; (3) pituitary responsiveness to a "physiologic" challenge with exogenous gonadotropin-releasing hormone (GnRH); (4) concentration of progestin receptors in cell cytosols and nuclei from the hypothalamus, preoptic area, and pituitary of rats treated with various classes of steroids to either inhibit or facilitate LH surges (to compare basal and estrogen-inducible progestin receptors in these tissues). Progestin receptors will be quantitated by in vitro exchange with tritiated high affinity synthetic progestins (R5020 or ORG 2058). Where appropriate intrapituitary LH content and serum concentrations of LH and GnRH will be measured by radioimmunoassay.